Incongruities between our projections and official databases were observed. According to the Ministry of Agriculture, Livestock, and Supply (MAPA), there were only four assignment contracts to use Brazilian waters for seaweed cultivation in 2020. Of those, only two of the assignees delivered the mandatory Annual Report of Production, and they declared in those documents that they had no production to report in 2020 (Brasil 2021). The incongruities observed are mainly related to legal regulations, as permission to use Brazilian waters for aquaculture falls under the purview of the Federal Government – while licensing is an attribute of each state. As such, and despite two decades of activity, the first commercial license for algaculture in Rio de Janeiro was only recently granted. Several other enterprises are still operative, however, as the government of Rio de Janeiro allows the continuity of those operations as long as the environmental license and the business licensing processes for marine farms have been protocolled.
Yield data was found to be difficult to compare objectively because of the different methodologies employed by different authors (Kasim and Mustafa 2017). Additionally, the date and time of image capture, weather, clouds, and other variables can influence image quality and therefore the ability to accurately evaluate cultivation structures. The methodology did, however, prove to be feasible for estimating algal biomass increases in standard culture structures (3m x 5m).
The algal growth values per square meter as estimated in this study (195–724%) are within those reported elsewhere (Kasim and Mustafa 2017). During experimental studies in southeast Sulawesi, Indonesia, weight gains after 40 days could reach 600% of the initial weight when using floating cages, and 300% with longlines (Kasim and Mustafa 2017). Similar results were obtained in Bongao, southern Philippines, where biomass increases of K. alvarezii can reach 300% after cultivation for 4 to 7 weeks (Villanueva et al. 2011). Fluctuations in K. alvarezii productivities and daily growth rates were associated with the positive effects of seawater temperature on growth (de Góes and Reis 2012).
Dos Santos and Hayashi (2022a) recently presented statistical data on K. alvarezii production in Santa Catarina State during the 2021/2022 season. Two cultivation methods were used, and 102.3 tons of algae were produced in an area of 3.20 ha – which represents 31.97 t/ha in only 2.2 production cycles. Biomass losses were also reported (10.3 to 21.3%), mainly due to high winds, sea currents, and the lack of experience of the algae farmers. The harvest information provided indicated a productivity of 58.12 t/ha in 4 production cycles under favourable conditions. Those values are within the range of values presented in scenarios 1 and 2, calculated based on daily growth rates between 1.5 and 3%.
The authors of that same study (dos Santos and Hayashi 2022a) estimated extract production from K. alvarezii (a bioproduct used as fertilizer); 800 ml of that bioproduct can be extracted from each kilogram of algae, and one liter can be sold for R$18.00. The 2021/2022 production in Santa Catarina was calculated to be 82,000 liters of extract, with a commercial value of 1.48 million reais (not considering associated costs). Using scenario 2, which estimated approximately 77 tons annually, 61,000 liters of extract could be produced, for total sales of 1.1 million reais.
The recorded parameters of temperature (Doty 1987; Glenn and Doty 1990; Hurtado et al. 2001; Ask and Azanza 2002; Paula and Pereira 2003; Munoz et al. 2004) and irradiance (Granbom et al. 2001) were within the range considered viable for K. alvarezii farming. The ideal growth conditions for most Brazilian algae species are between 22 and 28° C, with salinity levels between 28 and 36 ppm, although other species are more tolerant (Oliveira 1997). Depending on environmental temperatures, harvests may be greater or lesser, which will influence supply/demand and, consequently, the selling price (dos Santos and Hayashi 2022b).
There is also a positive correlation between algae growth and light levels (Guan et al. 2013; Lideman et al. 2013). According to Guan et al. 2013, K. alvarezii is photosynthetically adapted to low irradiance levels, but the species has adaptive mechanisms of acclimation to both excessive and deficient irradiance. Lideman et al. (2012) observed that the photosynthetic activity of a strain of K. alvarezii grown in Indonesia evidenced a high acclimation capacity in terms of environmental irradiance, decreasing its chlorophyll concentrations and increasing carotenoid levels with increasing depth (Indriatmoko et al. 2015); epiphytism, however, can be high in Sepetiba Bay during periods of warm water temperatures (Marroig and Reis 2011).
The irradiance level at which an alga is grown can directly influence its acclimation response, as well as growth rates and biomass gains. Terada et al. 2016 demonstrated that irradiance (associated with temperature) is one of the main factors stimulating or inhibiting photosynthetic activity and growth in K. alvarezii. The criterion of dissolved oxygen did not have a preponderant role in the weight of this sub-model, and no differences were observed between the two bays. The sewage pollution concentration map of the coastline between the municipality of Rio de Janeiro and Angra dos Reis (including almost the entire Sepetiba Bay) evidenced low suitability for seaweed cultivation.
Human populations in the region have been increasing, and several industries having high environmental impacts and located in the western part of Sepetiba Bay (in the municipalities of Rio de Janeiro and Itaguaí) contribute to the intense circulation of oil and mining vessels. Those corporations represent important sectors of the Brazilian economy – farming and mining (Rocha et al. 2012; Ribeiro et al. 2015; Gonçalves et al. 2020).
Total suspended solids were not found to be an important factor for seaweed cultivation. The existing cultivation structures in Ilha Grande Bay are floating rafts that maintain the algae very close to the surface (or just centimeters below it), where there is no theoretical influence of solids on water turbidity (de Góes and Reis 2012; Nogueira and Henriques 2020). Areas near the mouths of rivers, however, should be closely evaluated (Kjerfve et al. 2020). Carrageenan yields and quality are dependent of a number of different abiotic factors, and variations in carrageenan yields and viscosity in the seedlings of K. alvarezii and H. musciformis cultivated at Marambaia Bay probably reflected unfavorable environmental conditions (Hayashi et al. 2007b; Reis et al. 2008). The highest carrageenan yield observed by de Góes and Reis (2012) appeared to be a response to high rainfall.
Infrastructure support and logistics are the first and foremost aspects to consider in site evaluation. A lack of proper infrastructure resources reasonably near aquaculture facilities will increase costs and can cause initiatives to fail (Landuci et al. 2020). The large-scale commercial deployment of seaweed cultivation sites will also depend on skilled labor, equipment, drying, transportation, and water availability (Nogueira and Henriques 2020). Therefore, areas closer to certain inputs or services will provide strategic, logistical, and economic advantages. The production of large amounts of dry material requires healthy young forms of the algae to be cultivated, but commercial demands and product availability will, of course, influence selling prices (dos Santos et al. 2011; dos Santos and Hayashi 2022a). Several different cultivars of K. alvarezii, both experimental and commercial, are grown in the central area of the IGB, and the use of certified algae is a legal prerequisite for farming activity in Brazil. The nearby availability of different cultivars represents a cost-reducing advantage, improves survival rates during transport to other cultivation sites, and expedites responses in cases of urgency (Landuci et al. 2020).
Since cultivation requires nearby land-based facilities, accessibility represents a key production factor. An extension of the Rio-Santos highway passes a short distance from IGB coast, and that bay also has a high number of maritime support facilities as compared to Sepetiba Bay. Nautical transport of people and goods is common in the area, and the large numbers of vacationers to the area encourage this type of service. There are also shipyards, oil industries, and a granary port that require support structures (Landuci et al. 2020). Qualified labor is important for the success of cultivation efforts, and experienced workers living in the region will allow productivity gains and reductions of travel costs. Low quality labor can result in the increasing growth of calcareous organisms, which reduce irradiance – and therefore productivity (Marroig and Reis 2016).
The "Projeto de Algicultura" program, linked to the Federal University of Rio de Janeiro, located at Ilha Grande Bay, has been training specialized labor for the cultivation of Kappaphycus alvarezii (algicultura.com), and offers a strategic advantage to that region. In Brazil, agile management also reduces employment costs (de Góes and Reis 2011). Hurtado et al. 2001 noted that the highest outlays by some farms were related to labor costs (40%), followed by capital outlays (22%), materials (21%), and seedlings (12%). One carrageenan factory located near a legal cultivation area acquires intact tubular nets with the seedlings still inside them (de Góes and Reis 2011), resulting in considerable savings in terms of time and transportation of the product from farm to factory.
Technical support has helped define which cultivation techniques are most appropriate and generate the lowest costs to producers. Depending on the technique used, it is possible to reduce the hydrodynamic forces acting on cultivation structures and the amounts of nutrients that pass through the algae, as has been observed in various countries (Ask and Azanza 2002; de Góes and Reis 2011).
Hydrodynamism is important for algaculture as moving waters bring nutrients, but attention must be paid to the strength of this potential stressor (Marroig and Reis 2016). Sites experiencing high hydrodynamic forces tend to become dominated by calcareous organisms that reduce K. alvarezii production by decreasing solar irradiance, and strong currents can eventually cause the rafts to sink by hampering their cleaning (Marroig and Reis 2016). Coralline algae are more abundant in sites with greater hydrodynamic disturbance intensities (Marroig and Reis 2016). Water movements caused by meteorological changes (Castelar et al. 2009) can often break floating rafts, with consequent decreased productivity (de Góes and Reis 2012; Reis et al. 2015). Most cultivated areas are near small islands in deeper sites subject to winds and storms (Nogueira and Henriques 2020). Some studies, however, have reported greater abundances of epibionts on seedlings in areas having lower seawater movement, and lower abundances in areas with greater seawater movements (which tend to remove biofouling epibionts) (Hurtado et al. 2008; Vairappan et al. 2008).
The most frequent winds at Marambaia Bay are from the northeast (NE), northwest (NW), southwest (SW), and southeast (SE) (de Góes and Reis 2011), and floating rafts need to be placed perpendicular to the currents. It is important to follow cultivation protocols for the best productivity (Hayashi et al. 2007a), and poor raft position in relation to wind direction and intensity has been shown to diminish productivity by detaching seedlings. Environments with strong wave action and high winds are undesirable for growing eucheumatoids (Glenn and Doty 1990; Hurtado et al. 2001; Munoz et al. 2004).
Wind intensity can influence crop development in two ways: positively, when it increases hydrodynamics and consequently increases growth (and has been observed elsewhere and in Brazil; Doty 1987; Glenn and Doty 1990; Hayashi et al. 2010); and negatively, when there is breakage of the cultivation structures (floating rafts) with consequent seedling loss. Strong currents and sediment movement can scour, ensure adequate oxygen supplies, and reduce waste accumulation (Landuci et al. 2020). Sandy deposits are found at the entrance of STB, coinciding with the main channels, due to the strong influence of background currents. Muddy sediments predominate in the central portion, towards the shallower parts of the bay, due to lower intensity currents. Sandy sediments predominate in STB, however, which appears to be related to the presence of sand dunes along its banks and to SE winds.
Although IGB and STB are both well-protected, IGB has more protected sites. The southern coast of Rio de Janeiro is very rugged, with many coves, inlets and entrances. Flow is less intense in the innermost portion of IGB due to the many sheltered coves along the coastline. As a result, residence time is much higher in the northern end of the IGB bay system, close to the mainland, even considering the effects of meteorological tides. Tidal flows are more intense near the channels of communication with the ocean, reaching speeds of 50–75 cm/s. Water circulation in STB is clockwise, mainly determined by tidal forces (Landuci et al. 2020).
The bathymetry of IGB is characterized by extensive shallow areas with depths of up to 40 m; bathymetric dimensions can reach 55 m, however, in the main channels. The confined areas of smaller bays and coves have average depths < 10 m and represent 28.6% of the total IGB area (Landuci et al. 2020).
The highest of Human Development Index (H.D.I.) values are observed in the STB region, due to their proximity to the central markets of Rio de Janeiro. This pattern inverts as the distance from Rio de Janeiro increases, resulting in lower suitability in IGB, especially in the western zone close to the border with São Paulo State. The HDI is a summary measure of average achievement in terms of the significant dimensions of human development. The index was created to emphasize that a person and his/her capabilities should be the ultimate criteria for assessing the development of a country and indicates a potential to host more successful initiatives in areas with high grades, as its residents have better living conditions (Landuci et al. 2020).
It is fundamentally important to monitor the establishment of introduced seedlings in the environment as a preventive action for marine conservation (Castelar et al. 2009; Marroig and Reis 2011) The proximity of farming to protected areas is of significant concern because of our limited understanding of sea perimeters, rights, and responsibilities. There is a general respectful mentality in IGB regarding environmentally protected marine areas that is based on the fear of significant fines and penalties (Begossi et al. 2011).
Neither fisheries nor tourist activities are presently in conflict with mariculture in either bay. The region has intense fisheries that have shifted from traditional capture methods to the business sector, with the primary industrial fisheries employing trawling and purse seines (Landuci et al. 2020). Aquaculture is a tourist attraction in the region, and tourism is, in turn, an important consumer market for the aquaculture products of IGB and STB (and offers advantages related to reductions in transportation costs and improved market prices).
In general, the suitability model for both bays pointed out few unsuitable sites for seaweed farming and many areas with favorable environmental and geographical conditions. On the other hand, however, there are relatively few extremely suitable sites – mainly due to intense use of the coastline and conflicts between both existing heavy industries and conservation areas, especially in IGB. These results may initially seem disappointing due to the low numbers of highly suitable areas identified, but tourism and marine-related industries are the main sources of income for local populations in the densely developed south-eastern coast of Brazil (Landuci et al. 2020). The total suitable and available area (17,540 ha) is therefore sufficient to ensure the establishment and expansion of the marine algaculture industry along the south-eastern coast of Brazil, especially in Rio de Janeiro, where environmental operating permits are still waiting to be released.
Acquiring data with adequate resolution for detailed studies is an important consideration. The maps presented here are at regional scales, and some sites may not be evaluated in equal detail. Site selection requires geographically related data and information with multiple viable alternatives. Those alternatives are often conflicting, however, and may involve incompatible evaluation criteria. The suitability of a given site can also change over time, making it is important to consider short- and long-term issues that could result in overestimating or underestimating site availability (Landuci et al. 2020). The questionnaires used were designed to quickly and easily collect information from diverse social actors associated with the cultivation of marine macroalgae. Even so, it was often difficult to obtain responses from experts and the general public – although renowned national researchers in the area of marine aquaculture significantly contributed to the study.
Rio de Janeiro State demonstrates significant potential for the development of commercial aquaculture due to a number of positive characteristics, including favorable climatic conditions and large areas available to initiate such enterprises (Landuci 2021). Aquaculture can provide alternative income sources for coastal communities and, with proper planning, can avoid losses within the production chain (Landuci 2021). Additionally, macroalgae cultivation does not require fertilizer or pesticide inputs, and is independent of irrigation (Hargreaves 2013). If there is no major planning, capable executive management, or state intervention, however, those activities could result in irreversible environmental and social problems. While the cultivation of K. alvarezii was initiated more than 15 years ago, that activity is still undertaken on only a small scale (Valenti et al. 2021). Algaculture recently obtained the status of a strategic economic activity in the development of Rio de Janeiro State, and efforts are now focused on structuring the production chain.
The final model indicated Sepetiba Bay as the most suitable for aquaculture in light of logistics and costs, although that environment is known to suffer from heavy metal contamination (Rocha et al. 2010; Rocha et al. 2012; Souza et al. 2014) and some algae-derived products may not be viable for human consumption. Aquatic metal concentrations may be directly or indirectly affected by anthropogenic impacts such as acidification (which can increase trace metal availability). Salinity is often discussed with respect to metal accumulation by marine macroalgae.
The feasibility of using metal-contaminated macroalgae biomasses for bioenergy and bio-coal production in small-scale gasification facilities has been examined (Arena 2012; Schultz-Zehden and Matczak 2012). The main final product would be natural biogas used to generate electricity and heat and transportation fuels; the decomposition residues could be used as a soil fertilizer or for soil formation, and would improve soil functional redundancy, increase organic carbon stability, favor water retention, and support soil pH buffering capacities (Tammeorg et al. 2017; Hilber et al. 2017; Verheijen et al. 2017). In contrast to fossil coal, bio-coal is considered carbon-neutral as it is made from biomasses that accumulated their carbon contents directly from the atmosphere.
The ecosystem services provided by macroalgae are fully aligned with several Sustainable Development Goals, especially SDGs 2, 3, 5, 8, 10, 12, 13 and 14, which encompass environmental, social, and economic pillars of sustainability (Ferreira et al. 2021). Further research by environmental engineers, aquaculturists, and agriculturists can determine optimal cultivation times and environments for each macroalgae application. Inland tank cultivation, integrating multi-trophic aquaculture systems, could also provide effective alternatives for managing metals during macroalgae cultivation (Ratcliff et al. 2016).
Some of the challenges associated with pollution in open-water cultivation sites could be addressed through major engineering projects. Remote sensing technologies, long established in agriculture, may soon allow aquaculture farmers to evaluate numerous water parameters in coastal environments and identify potential open-water sites. Furthermore, a greater understanding of accumulation capacities could help remediation efforts in polluted areas and assist future designs of biological remediation systems in marine settings.
The cultivation of macroalgae can also have a role in remediating the effects of climate change, especially global warming and rising CO2 rates in the atmosphere, due to their great photosynthetic and atmospheric carbon sequestration capacities (Chung et al. 2011; Chung et al. 2010). In the case of red macroalgae, such as K. alvarezii, their different photosynthetic pigments will be present in different concentrations due to variations in light intensity at different depths (Indriatmoko et al. 2015).